HSBC has come under a cyber attack - but said customers' details have not been compromised.
The bank said its personal banking website was affected by a "denial of service attack", which it has successfully defended itself against.
A spokesman for HSBC said: "HSBC internet banking came under a denial of service attack this morning, which affected personal banking websites in the UK.
"HSBC has successfully defended against the attack, and customer transactions were not affected.
"We are working hard to restore services, and normal service is now being resumed.
"We apologise for any inconvenience this incident may have caused."
Earlier this month, HSBC apologised to customers after it suffered online banking glitches for two days running.
Those problems, which happened at the start of January, were put down to a "complex technical issue" with its internet banking systems and not a cyber attack or any other malicious act.
The latest problems have arrived on what is payday for many people.
People vented their frustration at the latest internet problems on Twitter.
One user said: "Cannot believe @HSBC online banking is STILL down...seriously?!"
Another Twitter user said: "So #HSBC online banking is down ... again! Guess I should just re-read the apologetic letter I rec'd only last week re: the previous outage!"
The attack also comes just days before the self-assessment deadline for tax returns to be submitted and any tax owing paid, which is on Sunday.
A denial of service attack happens when hackers attempt to prevent people from using a service by bombarding it with traffic.
HSBC's website was displaying an error message, saying: "Sorry, there appears to be a system problem. Please try again later."
Another Twitter user complained that customer services had earlier appeared not to know about the cyber attack, saying they were asking people having problems logging in to "clear their caches and cookies".
The issues with HSBC follow the high-profile cyber attack last year involving telecoms giant TalkTalk, which resulted in customers' personal details being accessed.